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how big they get is in their genetics, and we can't control that unless you stunt their growth w/ a small tank.

I think you are overfeeding them. fish, and even moreso with gold fish. eat as much as they can and can eat themselves to death. they still have that wild mentality that they don't know when food is coming next so eat as much as possible when they see it.

i feed my fish every other day and once a day.
they are happy and very healthy.

Feeding adult fish is one of those things where a rule of thumb actually applies. Once a day and never more than is consumed in 60 seconds. only with young or very, very small (1 inch or less) fish are you better off doing 2 45 second feeds.

Most fish are perpetually hungry because, as already stated, the never has been a need for a "full" sensation due to food scarcity in the wild. However, as you become experienced you will recognize two types of hungry. One is the "hungry" hungry which occurs with true need for food. The other is the "not so hungry" hungry where a fish will continue eating just simply because the still is food present. With experience you will be able to differentiate both forms of "hungry" and stop feeding accordingly.